In order to compare our results with previous studies, we applied a Gaussian toy model to the Na D doublet region of our transmission spectrum to estimate the strength of the absorption signal.
We used the sum of two Gaussian profiles centered on the lines of the Na D doublet and applied a Markov chain Monte Carlo (MCMC) procedure using emcee [1]} to determine the best model parameters and their uncertainties. The details of this procedure including the prior and posterior values are described in Appendix .
